Neutron spectrometer

ABSTRACT

A neutron spectrometer is provided by a series of substrates covered by a solid-state detector stacked on an absorbing layer. As many as 12 substrates that convert neutrons to protons are covered by a layer of absorbing material, acting as a proton absorber, with the detector placed within the layer to count protons passing through the absorbing layer. By using 12 detectors the range of neutron energies are covered. The flat embodiment of the neutron spectrometer is a chamber, a group of detectors each having an absorber layer, with each detector separated by gaps and arranged in an egg-crate-like structure within the chamber. Each absorber layer is constructed with a different thickness according to the minimum and maximum energies of neutrons in the spectrum. In this arrangement, each of the 12 surface facets provides a polyethylene substrate to convert neutrons to protons, covered by a layer of absorbing material, acting as a proton absorber, with the detector stacked on the absorbing layer to count protons passing through the absorbing layer.

DIVISIONAL APPLICATION

This application is a divisional application of U.S. Patent OfficeApplication Ser. No. 09/503,858, entitled “Dodecahedron NeutronSpectrometer,” which was filed on Feb. 14, 2000 now U.S. Pat. No.6,349,124, by the inventors herein. This divisional application is beingfiled under 35 USC §120 and 37 CFR § 1.53, and priority from thatapplication is hereby claimed.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, sold,imported, and/or licensed by or for the Government of the United Statesof America without the payment to us of any royalties thereon.

FIELD OF INTEREST

The invention relates to radiation sensors and, more particularly, to aspectrometer to measure an unknown neutron spectrum in outer space.

BACKGROUND OF THE INVENTION

It is often necessary to quickly, accurately and inexpensively measureneutron spectra in low earth orbits covering several energy ranges.High-energy cosmic rays produce neutrons in the upper atmosphere are aparticular concern because such neutrons pose a threat to airbornesemiconductor equipment such as the memory devices in flight controlequipment. Neutrons threaten these devices by causing bit-flips leadingto failures in the flight control and navigational equipment, andthereby endangering the operation of both high-flying aircraft like theConcorde and lower altitude commercial aircraft.

There has been a long-standing need to characterize neutron spectra sothat physicists and equipment designers can better predict aircraftupset rates and design systems to avoid catastrophic aircraft failures.The general operating principle for neutron spectrometers is thatneutrons interact with certain atoms to produce recoil protons thattravel in relatively straight lines, as described in Kronenberg, S. andH. Murphy, “Energy Spectrum of Protons Emitted From aFast-Neutron-Irradiated Hydrogenous Material”, Radiation Research 12,728-735 1960.

Several types of detectors that have been used in prior art neutronspectrometers of this type to measure the recoil protons. One of theearliest applications described in Kronenberg, S., “Fast NeutronSpectroscope for Measurements in a High Intensity Time Dependent NeutronEnvironment”, International Symposium on Nuclear Electronics”, ParisFrance, Comptes Rendus, May 1964. That device utilized a scintillationcounter, consisting of cesium iodide and a photomultiplier and solidstate devices. A variation of that approach employing a PMOS transistorwas described in Kronenberg, S. and G. J. Brucker, “The Use ofHydrogenous Material for Sensitizing PMOS Dosimeters to Neutrons”, IEEETrans. Nucl. Sci., Vol. 42, No. 1, February 1995.

One significant limitation of these prior art devices is that they canonly count protons and can neither characterize neutron spectra norgenerate the original neutron spectra. These prior art neutronspectrometers suffered from a number of other disadvantages, limitationsand shortcomings because of their size, weight cost and complexcircuitry, making them unsuitable for use in spacecraft and otherairborne applications. In fact, the NASA Goddard Space Flight Centerrecently requested proposals for the measurement of high-energy spectrawith a spectrometer on-board a satellite or the Shuttle spacecraft.

To overcome the prior art's inability to characterize neutron spectra,as well as disadvantages, limitations and shortcomings of size, weight,cost and complex circuitry, the present invention fulfills thislong-standing need with a simplified, compact and inexpensive neutronspectrometer detector. The neutron spectrometer detector employs a thindepletion layer, silicon, solid state detector as a proton counter in aninstrument that converts a distribution of neutrons to one of recoilprotons. The present invention's neutron spectrometer uses computertechnology to allow for greater and quicker data reduction and providesthe added capability of characterizing neutron spectra by unfoldingproton recoil spectra into the original neutron spectrum that producedthe proton particles.

The preferred embodiment is flat neutron spectrometer monitor with anarrangement of detectors, converters and absorbers housed within achamber. The advantages of low weight, compact size, simplifiedoperation and increased data reduction allow the present invention'sneutron spectrometer to fulfill the long-standing need for measuringhigh-energy spectra, without suffering from the disadvantages,limitations and shortcomings of prior art devices. A dodecahedronembodiment of the neutron spectrometer with the detectors, convertersand absorbers housed within a sphere is also described.

SUMMARY OF THE INVENTION

It is one object of the neutron spectrometer to measure neutron spectraon land or in the laboratory.

It is another object of the neutron spectrometer to measure neutronspectra covering several energy ranges from 1 to 250 MeV.

It is an additional object of the neutron spectrometer to convert adistribution of neutrons to one of recoil protons sorted into numerousenergy bins where they are counted and the original neutron spectrum isgenerated by software.

To attain these and other objects and advantages, the neutronspectrometer of the present invention provides a series of substratescovered by a solid-state detector stacked on an absorbing layer. In thisarrangement, as many as 12 substrates that convert neutrons to protons,are covered by a layer of absorbing material, acting as a protonabsorber, with the detector placed within the layer to count protonspassing through the absorbing layer. By using 12 detectors the presentinvention covers the range of neutron energies. The present inventionencompasses a preferred dodecahedron spectrometer, and other shapes arealso possible.

The dodecahedron embodiment of the present invention's neutronspectrometer comprises a solid, polyethylene dodecahedron assembly withits 12 surface facets covered by a solid-state detector stacked on anabsorbing layer. In this arrangement, each of 12 surface pentagon-shapedfacets provides a polyethylene substrate to convert neutrons to protons,covered by a layer of absorbing material, acting as a proton absorber,with the detector stacked on the absorbing layer to count protonspassing through the absorbing layer. The dodecahedron assembly is housedconcentrically within a titanium spherical shell that serves as an outershield. The dodecahedron embodiment is lightweight and therefore wouldbe suitable for airborne and satellite applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual cross section view of a neutron detector.

FIG. 2 is a chart that shows plots of counts in the detector versusproton energy with different thicknesses indicated as a parameter on thecurves.

FIGS. 3A and 3B are perspective views of a neutron detector and adodecahedron neutron spectrometer.

FIG. 4 is a front view drawing of the dodecahedron neutron spectrometerwith representative dimensions.

FIG. 5 is a perspective drawing of the dodecahedron neutron spectrometerremoved from the shell depicting absorbing layers of varying thickness.

FIG. 6 is a perspective conceptual drawing of the flat neutronspectrometer of the present invention.

Table I is a listing of absorbing layer thicknesses.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1, the essential principle of operation for thedevices of the present invention is illustrated. FIG. 1 is a conceptualcross section view of a single neutron detector comprising a means fordetecting neutrons 10 stacked on an absorbing layer, or proton-absorbinglayer, 11. The absorbing layer 11, being composed of a first materialthat absorbs protons, such as titanium, is stacked on a hydrogenoussubstrate 12. Hydrogenous substrate 12 is composed of a second materialhaving hydrogen atoms interacting with an unknown source of neutrons,indicated by box 13.

When a single neutron detector is placed in a field of a neutronspectrum, the incident neutrons, indicated by arrow 14, from suspectedneutron source 13 interact with hydrogen atoms within hydrogenoussubstrate 12. This interaction produces proton recoils that travel infairly straight lines, one of which is indicated by arrow 15, throughthe absorber layer 11 and the detector means 10. Scattered neutrons,indicated by arrow 16, are deflected away from the hydrogenous substrate12. Detector means 10 is connected to a data processing means, indicatedby box 17, and a ground 18. The data processing means 17 includes ameans for proton distribution. Using several detector means 10 with eachabsorbing layer 11 having a different thickness allows protons withenergies and corresponding ranges greater than the thickness of aparticular absorbing layer 11 to reach detector means 10 and produceproton counts. The amount of absorber layers 11 and their thickness canbe selected to correspond to ranges of protons from a low value for 1MeV and larger thicknesses of 250 MeV. Hydrogenous substrate 12 convertspart of the kinetic neutron energy to energy of the recoil protons 15and the detector means 10 detects protons passing through the absorbinglayer 11.

This approach is demonstrated by considering the energy transferbehavior of neutrons and protons. The maximum energy a neutron of energyE_(n) can transfer to a proton E_(p) (max) equals E_(n) (1,2). For thisexample, assume an absorbing layer 11 thickness of d. For monoenergeticneutrons (E_(n)) the number of recoil protons reaching detecting means10 and producing proton counts decreases as energy E_(n) decreases. Thenumber of protons will eventually equal zero when the range of maximumenergy recoil protons becomes smaller than d. Recoil particles due toelastic scattering do occur in the higher atomic number non-hydrogenousabsorber but, except for very high E_(n), they do not contribute to thecounts due to their small range and the unfavorable quantum energytransfer in elastic scattering.

Having a system with K units, each with a different d and exposing themto a neutron spectrum, one obtains data which consist of K counts orcount rate values C_(i)(d_(i)) i=1, 2, . . . K where ford_(i−1)<d_(i)<d_(i+1), C_(i−1)(d_(i−1))>C_(i)>C₊₁. From these numbersone can unfold the incident spectrum of neutrons.

The detector means 10 can be of any shape or configuration and can beany type of solid state device. The inventors herein have employed adepleted n/p diode used to measure alpha particles, which was relativelyinsensitive to beta particles because of their low LET (Linear EnergyTransfer) values as a detector means 10. Spectroscopic grade detectorsare not required for this device since only event counting is requiredand data describing the energy spectrum are not needed.

In considering the thicknesses of absorbing layers 11 and the ranges ofprotons to be measured, an energy range of 1 to 250 MeV was selected tomatch the expected neutron spectrum distribution. One solution toachieve this objective is to fabricate an instrument that converts adistribution of neutrons to one of recoil protons, which are chargedparticles that can be easily counted. By employing 12 detector means 10within a given chamber, the recoil protons are essentially sorted into12 bins where they can be readily counted. Said absorber layers 11 canbe constructed of aluminum for detecting the lower energy levels ortantalum for the higher values. The hydrogenous substrate 12 for eachdetector means 10 could be constructed of polyethylene.

The data processing means 17 and its means for proton distributionprovides a hitherto unavailable capability to determine a protondistribution pattern to construct a neutron spectrum indicating thespectrum of neutrons from an unknown source of neutrons 13. Inoperation, results of a spectral measurement are a set of pairs from thedetector means 10 and the absorbing layer 11 that allows protons withenergies and corresponding ranges greater than the absorbing layer 11'sthickness to reach the detector means 10 and produce proton recoilcounts. One data processing means 17 successfully employed by thepresent inventors is a 3-dimensional Monte Carlo Adjoint Transport code,NOVICE, which is described in Jordan, T., “Novice, A Radiation Transportand Shielding Code”, Experimental and Mathematical Physics Consultant,Report EMP. L 82.001, January 1982.

FIG. 2 is a chart showing plots of counts in the detector versus protonenergy with different thicknesses indicated as a parameter on thecurves, and these results were obtained using the NOVICE program and aflat spectrometer 20 depicted in FIG. 6, which will be described below.The FIG. 2 plots are counts in the detector versus proton energy withthe aluminum and tantalum thicknesses indicated as a parameter on thecurves. In this preliminary assessment of the feasibility of neutronmonitor with multiple neutron detectors, an incident neutron spectrumand the subsequent unfolding software were not included in the code'srun. The proton recoil spectrum was assumed to exist in the convertermaterial of hydrogenous substrate 12. The separation or resolution ofproton energy shown in FIG. 2 provides useful information aboutdetecting 12 ranges of neutron energy. The flat configuration of monitor20, depicted in FIG. 6, along with the use of tantalum for the absorberlayers 11 and for the chamber 21 make it too heavy for spacecraft orother airborne applications. Using a data processing device with theNOVICE computer software to analyze the monitor revealed other moreuseful potential configurations for neutron spectrometers, which weremodeled and analyzed by the computer.

One configuration suggested by the FIG. 2 NOVICE results is a pentagondodecahedron, which allows for a full measurement range because of its12 surfaces, each supporting a detector-absorber pair with differentabsorber layer thicknesses. FIGS. 3A and 3B, are perspective drawingsdepicting a detector means 41 stacked on a pentagonal absorbing layer 42and a dodecahedron. neutron spectrometer monitor 40, respectively.

Referring now to FIG. 3A, which depicts a perspective view of a neutrondetector comprising a detector means 41 stacked on an absorbing layer42. Absorbing layer 42 is composed of a first material that absorbsprotons, such as titanium. By placing this assembly on an appropriatehydrogenous substrate, a neutron detector is provided. Referring now toFIG. 3B, dodecahedron neutron spectrometer monitor 40 is depicted with11 of 12 of the absorbing layers 42 with varying thicknesses stacked ona surface facet of a solid dodecahedron substrate 43, which provides thehydrogenous substrate. Dodecahedron substrate 43 is shown partiallyexposed without one absorbing layer for illustrative purposes.

FIG. 4 is a front view drawing of the dodecahedron neutron spectrometermonitor 40 with all absorbing layers 51-62, respectively, covering eachof the 12 facets of substrate 43 and representative dimensions. For thesake of clarity, only one detector means 41 is shown stacked onabsorbing layer 54, with 11 other detector means 41 for the other 11absorbing layers 51-53 and 55-62, respectively, not shown. Each of the12 absorbing layers 51-62 are constructed with a varying thickness andare stacked on a surface facet of the solid dodecahedron substrate 43.Substrate 43 is composed of a hydrogenous material, such aspolyethylene, having hydrogen atoms and functions as a neutron converterwhen interacting with said absorbing layers 51-62 in the presence of allunknown energy distribution, indicated by box 44, which emits incidentneutrons, indicated by arrow 63.

In operation, said hydrogenous substrate 43 converts said neutrons torecoil protons and each of said detector means 41 detects recoil protonspassing through each absorbing layer 51-62, respectively. Each absorbinglayer 51-62, respectively has a different thickness, as depicted in FIG.5, to absorb neutron energies from 1 to 250 MeV. Returning now to FIG.4, the hydrogenous substrate 43 is housed in a concentrically hollowspherical chamber, indicated by broken line 45. Each detector means 41is coupled to a means for data processing, indicated by box 46, outsidethe spherical chamber 45, which provides a count of recoil protons to ameans for proton distribution, not shown, residing within said dataprocessing means 46. The means for proton distribution determines aproton distribution pattern to construct a neutron spectrum patternindicating the spectrum of neutrons from said suspected source ofneutron radiation 44.

FIG. 4 also includes representative dimensions. Each absorbing layer51-62 is pentagonally shaped in this embodiment, with each side 2.03 cmin length. Each of said detector means 41 are circular and 0.5″ wide and0.015″ thick. Covered hydrogenous substrate 43 is 4.47 cm in height andhoused concentrically within hollow spherical chamber 45. Hydrogenoussubstrate 43 was fabricated from a solid block of Lucite™. The hollowspherical chamber 45 is composed of titanium in this embodiment with aninner diameter of 10.8 cm and a wall thickness of 2.5 cm. Each of said12 absorbing layers 51-62 is composed of titanium in this embodimentwith a varying thickness ranging from 0.00105 cm to 2.4217 cm, asdescribed in Table I below.

TABLE I ABSORBING LAYER THICKNESS ENERGY THICKNESS ABSORBER (Mev's)FACET (cm's) 51 1.0 10 .00105 52 1.5 3 .00191 53 2.5 4 .00425 54 4.0 6.00911 55 8.5 7 .02051 56 10.0 8 .04271 57 15.0 9 .08606 58 25.0 11.21027 59 40.0 12 .48153 60 65.0 5 1.1353 61 80.0 2 1.6369 62 100.0 12.4217

Detector means 41 can be constructed from a depleted n/p diode. Itshould be understood to skilled in the art that these dimensions aremerely representative and numerous other choices of dimensions arepossible.

FIG. 5 is a perspective drawing of hydrogenous substrate 43, using likenumerals for similar structural elements, illustrating a number ofabsorbing layers with a varying thickness. In this drawing, coveredhydrogenous substrate 43 is shown removed from the hollow sphericalshell 45 to better illustrate each absorbing layer having a differentthickness.

Referring back to FIG. 2, which is the chart showing plots of counts inthe detector versus proton energy with different, thicknesses indicatedas a parameter on the curves from the NOVICE program. Those plots fromthe FIG. 6 flat spectrometer 20, which will be described shortly, arebased on using aluminum and tantalum as absorber material. These resultssuggested using titanium as the preferred absorber material for the FIG.4 absorbing layers 51-62 for all energy levels, because titanium islighter than tantalum and its neutrons do not generate nuclearinteractions. Only elastic scattering takes place. The proton energyresolution from this embodiment is also relatively good. The FIG. 2results also indicate that aluminum absorbers produced a slightly betterenergy resolution for the lower range of energies, 1 to 10 MeV. The sizeof this dodecahedron configuration is small and light in weight and verypractical for a spacecraft application.

In order to insure that an unknown neutron spectrum has an isotropicdistribution, the spectrometer 40 can also be located at the center of atitanium sphere with a diameter of 3 inches.

FIG. 6 is a perspective conceptual drawing of the flat embodiment of thepresent invention's neutron spectrometer monitor 70. Monitor 70comprises a group of the FIG. 1 neutron detector means 10 arranged in achamber 71. As described above, having several detector means 10 stackedonto absorbing layers, not shown, each having a different thickness,allows protons with energies and corresponding ranges greater than thethickness of each absorbing layer to reach the detector means 10 andproduce proton counts. FIG. 6 depicts 12 detector means 10 whichcorrespond to 12 energy bins and thus detect protons with rangescorresponding to energies from 1 MeV up to 250 MeV. The floor of chamber71 serves as the hydrogenous substrate. Monitor 70 is placed inproximity to an unknown source of neutrons, shown as box 76.

Detecting means 10 is coupled to a means for data processing, indicatedby box 77, and provides a separate count of recoil protons for eachdifferent thickness employed in the absorbing layers. The dataprocessing means 77 transmits the count of recoil protons to a means forproton distribution, not shown, residing within the data processingmeans 77. The means for proton distribution determines a protondistribution pattern to construct a neutron spectrum pattern indicatingthe spectrum of neutrons from the suspected concentration of neutrons76. Bulkhead output connector 72 on the chamber 71 allows correction ofvoltage to the detector as well as correction of output counts tocounting instruments.

In the flat configuration, said chamber 71 is shown in a rectangularshape, and its walls 78, lid, not shown, and unit compartments 79 can becomposed of tantalum. Each detector means 10 in the egg-crate-likestructure is numbered 1′-12′, respectively, to correspond with readingsshown in the FIG. 2 chart. Detector means 7′ is depicted withrepresentative dimensions of 2 cm in width and 2 cm in length. A gap 80between detector means 11′ and 12′ is 0.471 cm. The thickness of eachwall 78 is 1 cm and its height is about 3 cm. The chamber 71 is depictedas 15 cm in length and 5.41 cm in width. These dimensions are merelyrepresentative and numerous other choices of dimensions are possible,however, it is critical that each absorber layer is constructed with adifferent thickness according to the minimum and maximum energies ofneutrons in the spectrum. Similarly, the materials used for constructingthe absorber layers, detector means 10 and chamber 71 can also be variedaccording to the minimum and maximum energies of neutrons in thespectrum.

It is to be understood that details concerning materials, shapes anddimensions are merely illustrative, and that other combinations ofmaterials, shapes and dimensions can also be advantageously employed andare considered to be within the contemplation of the present invention.We also wish it to be understood that we do not desire to be limited tothe exact details of construction shown and described. It will beapparent that various structural modifications may be made withoutdeparting from the spirit of the invention and the scope of the appendedclaims.

What we claim is:
 1. A neutron spectrometer monitor, comprising: aplurality of neutron detectors; said monitor is placed in proximity to asuspected concentration of neutron radiation; each of said plurality ofneutron detectors further comprising a detector means stacked on atantalum proton-absorbing layer, each of said proton-absorbing layersbeing stacked on a hydrogenous substrate; said hydrogenous substratebeing composed of polyethylene and containing hydrogen atoms, saidhydrogen atoms interacting with said suspected concentration of neutronradiation, said hydrogenous substrate converting said neutron radiationto a plurality of recoil protons that travel in straight lines throughsaid proton-absorbing layer and said detector means, each of saiddetector means detecting said plurality of recoil protons and furthercomprising a depleted n/p diode; said hydrogenous substrate deflecting aplurality of scattered neutrons away from said hydrogenous substrate;each of said proton-absorbing layers having a different thickness, d, toabsorb a plurality of neutron energies from 1 to 250 MeV; said pluralityof neutron detectors being housed in a flat rectangular chamber composedof tantalum, said chamber having a polyethylene floor, a plurality ofcompartments for each of said detector means and a lid; each of saiddetector means, being coupled to a means for data processing, sends aseparate count of recoil protons for each of said different thicknesses,d, to said data processing means; said data processing means providingsaid separate count of recoil protons to a means for protondistribution; and said means for proton distribution determines a protondistribution pattern to generate a neutron spectrum pattern thatconstructs an original neutron spectrum from said suspectedconcentration of neutron radiation.
 2. The neutron spectrometer monitor,as recited in claim 1, further comprising: said plurality of recoilprotons reaching said detecting means and producing said separate countof recoil protons that decreases as a neutron energy, E_(n), decreases;said separate count of recoil protons decreases to zero when a range ofmaximum energy recoil protons becomes smaller than said differentthickness, d, and; said plurality of proton-absorbing layers, furthercomprising K number of proton-absorbing layers, each of said K number ofproton-absorbing layers having said different thickness, d, beingexposed to said suspected concentration of neutrons, provides a countrate calculated according to the formula: K count rate values C _(i)(d_(i))i=1, 2, . . . K where for d _(i−1) <d _(i) <d _(i+1) , C _(i−1)(d_(i−1))>C _(i) >C _(i+1).
 3. The neutron spectrometer monitor, asrecited in claim 2, further comprising said plurality of neutrondetectors having at least 12 of said detector means.
 4. The neutronspectrometer monitor, as recited in claim 3, further comprising saidpolyethylene being solid.